1. Field of the Invention
The present invention relates to an optical modulator, and in particular to an optical SSB modulator in which an asymmetrical X-branching is provided to output portions of sub Mach-Zehnder (MZ) waveguides composing the optical SSB modulator, thereby appropriately adjusting a phase of light in the MZ waveguides.
2. Description of the Related Art
In a wavelength multiplexing optical communication system of the next generation, it is expected that a switch-over between wavelength channels is performed in a node. Therefore, a wavelength conversion apparatus is required in the wavelength multiplexing optical communication system. As a wavelength conversion apparatus, an optical single side-band (SSB) modulator is known. The optical SSB modulator is an optical modulator which can obtain an output light having shifted for a frequency of a modulating signal (e.g. as described in [S. Shimotsu, S. Oikawa, T. Saitou, N. Mitsugi, K. Kubodera, T. Kawanishi and M. Izutsu, “Single Side-Band Modulation Performance of a LiNbO3 Integrated Modulator Consisting of Four-Phase Modulator Waveguide,” IEEE Photon. Tech. Lett., Vol. 13, 364–366 (2001)] and [Shimotsu Shinichi, Masayuki Izutsu, “LiNbO3 optical single-sideband modulator for next-generation communication”, Optical Alliance, 2000.7. pp. 27–30]).
FIG. 7 is a schematic diagram showing a basic arrangement of an optical SSB modulator. As shown in FIG. 7, an optical SSB modulator 101 is provided with a first sub Mach-Zehnder waveguide (MZA) 102, a second sub Mach-Zehnder waveguide (MZB) 103, a main Mach-Zehnder waveguide (MZC) 104, a first bias adjustment electrode (DCA electrode) 105, a second bias adjustment electrode (DCB electrode) 106, a first modulation electrode (RFA electrode) 107, a second modulation electrode (RFB electrode) 108 and a third bias adjustment electrode (DCC electrode) 109.
The main Mach-Zehnder waveguide (MZC) 104 is a Mach-Zehnder waveguide including the MZA and MZB as both of its arms.
The first bias adjustment electrode (DCA electrode) 105 is an electrode for controlling a bias voltage between two arms (Path 1 and Path 3) composing the MZA, thereby controlling a phase of light transmitted through the two arms of the MZA. On the other hand, the second bias adjustment electrode (DCB electrode) 106 is an electrode for controlling a bias voltage between two arms (Path 2 and Path 4) composing the MZB, thereby controlling a phase of light transmitted through the two arms of the MZB.
The first modulation electrode (RFA electrode) 107 is an electrode for inputting a radio frequency (RF) signal to the two arms composing the MZA. On the other hand, the second modulation electrode (RFB electrode) 108 is an electrode for inputting the RF signal to the two arms composing the MZB.
The third bias adjustment electrode (DCC electrode) 109 is an electrode for controlling the bias voltages of the MZA and the MZB, thereby controlling the phase of light transmitted through the MZA and the MZB. The third bias adjustment electrode (DCC electrode) is usually a direct current electrode or a low frequency electrode.
FIG. 8 is a schematic diagram showing an optical spectrum at each point of the optical SSB modulator of FIG. 7 in case an upper sideband is generated. At point P and point Q of FIG. 7, light of both sidebands is present. However, as for the output light, components (light of lower sideband in FIG. 8) whose phases are reversed at point P and point Q cancel each other, so that only the light of the single sideband (upper sideband in FIG. 8) is outputted.
In an optical SSB modulator, in order to obtain the above-mentioned output, sinusoidal RF signals with phases different from each other by 90° are inputted to four optical phase modulators paralleled, and bias voltages to be impressed to the bias adjustment electrodes (DCA electrode, DCB electrode and DCC electrode) are adjusted so that mutual phase differences are respectively 90° with respect to light. When this is done, light having an optical frequency shifted as much as the frequency (fm) of the modulating signal is obtained as an output. Directions of frequency shifting (whether the frequency is increased or decreased by fm) can be selected by setting the bias voltage to be impressed to the DCC electrode.
More specifically, a bias voltage (DCA) impressed to the DCA electrode is controlled so that the light phase difference between Path 1 and Path 3 of FIG. 7 assumes 180°. Also, a bias voltage (DCB) impressed to the DCB electrode is controlled so that the light phase difference between Path 2 and Path 4 assumes 180°. Then, a bias voltage (DCC) impressed to the DCC electrode is controlled so that the light phase difference between the two sub MZ waveguides assumes 90°.
It is to be noted that the operation of an conventional optical SSB modulator is described in detail in for example, [Tetsuya Kawanishi, Masayuki Izutsu, “Optical frequency shifter using optical SSB modulator”, TECHNICAL REPORT OF IEICE, OCS2002-49, PS2002-33, OFT2002-30 (2002–08)] and [Higuma et al., “X-cut lithium niobium optical SSB modulator, Electron Letter, vol. 37, 515–516 (2001)].
In the conventional optical SSB modulator, the bias voltage has been adjusted as follows in order to obtain a phase of light in the arm of each MZ waveguide described above. Namely, voltages impressed to the DCA electrode and the DCB electrode are slightly adjusted so that the output from the third MZ waveguide is minimized. Thereafter, voltages are impressed to the modulation electrodes, and a voltage impressed to the DCC electrode is slightly adjusted so that unnecessary contents included in the output light are minimized. Occasionally, the voltages impressed to the modulation electrodes are slightly adjusted. Such operations were repeated. However, there are cases in which the output from the third MZ waveguide becomes 0 even when the optical phases in both arms of the waveguides (MZA 102 and MZB 103) are not mutually different by 180°. Therefore, there has been a problem that the phase of light cannot always be appropriately adjusted with such a voltage adjustment method.
Also, in the conventional optical SSB modulator, light other than the output light such as the light of the single sideband (light of lower sideband in FIG. 8) that is not outputted due to mutual cancellation has leaked out of the modulator from the circuit. There has been a problem that such a leaked light degrades the performance of the optical SSB modulator.
Moreover, in the conventional optical SSB modulator, fine adjustments are repeated by observing the output light, so that there has been a problem that the voltage impressed to the bias adjustment electrode cannot be automatically adjusted, e.g. as described in Shimotsu Shinichi, Masayuki Izutsu, “LiNbO3 optical SSB modulator for next-generation communication”, Optical Alliance, 2000.7. Pp. 27–30.